bpo-29822: Make inspect.isabstract() work during __init_subclass__. (#678)

At the time when an abstract base class' __init_subclass__ runs,
ABCMeta.__new__ has not yet finished running, so in the presence of
__init_subclass__, inspect.isabstract() can no longer depend only on
TPFLAGS_IS_ABSTRACT.

2 files changed, 46 insertions, 1 deletions

diff --git a/Lib/inspect.py b/Lib/inspect.py
index a2dcb888a0..2894672f50 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -31,6 +31,7 @@ Here are some of the useful functions provided by this module:
 __author__ = ('Ka-Ping Yee <ping@lfw.org>',
               'Yury Selivanov <yselivanov@sprymix.com>')
 
+import abc
 import ast
 import dis
 import collections.abc
@@ -291,7 +292,27 @@ def isroutine(object):
 
 def isabstract(object):
     """Return true if the object is an abstract base class (ABC)."""
-    return bool(isinstance(object, type) and object.__flags__ & TPFLAGS_IS_ABSTRACT)
+    if not isinstance(object, type):
+        return False
+    if object.__flags__ & TPFLAGS_IS_ABSTRACT:
+        return True
+    if not issubclass(type(object), abc.ABCMeta):
+        return False
+    if hasattr(object, '__abstractmethods__'):
+        # It looks like ABCMeta.__new__ has finished running;
+        # TPFLAGS_IS_ABSTRACT should have been accurate.
+        return False
+    # It looks like ABCMeta.__new__ has not finished running yet; we're
+    # probably in __init_subclass__. We'll look for abstractmethods manually.
+    for name, value in object.__dict__.items():
+        if getattr(value, "__isabstractmethod__", False):
+            return True
+    for base in object.__bases__:
+        for name in getattr(base, "__abstractmethods__", ()):
+            value = getattr(object, name, None)
+            if getattr(value, "__isabstractmethod__", False):
+                return True
+    return False
 
 def getmembers(object, predicate=None):
     """Return all members of an object as (name, value) pairs sorted by name.
diff --git a/Lib/test/test_inspect.py b/Lib/test/test_inspect.py
index 2f12e989d9..799a9faa78 100644
--- a/Lib/test/test_inspect.py
+++ b/Lib/test/test_inspect.py
@@ -229,6 +229,30 @@ class TestPredicates(IsTestBase):
         self.assertFalse(inspect.isabstract(int))
         self.assertFalse(inspect.isabstract(5))
 
+    def test_isabstract_during_init_subclass(self):
+        from abc import ABCMeta, abstractmethod
+        isabstract_checks = []
+        class AbstractChecker(metaclass=ABCMeta):
+            def __init_subclass__(cls):
+                isabstract_checks.append(inspect.isabstract(cls))
+        class AbstractClassExample(AbstractChecker):
+            @abstractmethod
+            def foo(self):
+                pass
+        class ClassExample(AbstractClassExample):
+            def foo(self):
+                pass
+        self.assertEqual(isabstract_checks, [True, False])
+
+        isabstract_checks.clear()
+        class AbstractChild(AbstractClassExample):
+            pass
+        class AbstractGrandchild(AbstractChild):
+            pass
+        class ConcreteGrandchild(ClassExample):
+            pass
+        self.assertEqual(isabstract_checks, [True, True, False])
+
 
 class TestInterpreterStack(IsTestBase):
     def __init__(self, *args, **kwargs):